There has been needed a positioning device (stage) provided in a scanning electron microscope (SEM), for carrying thereon a sample, which requires a high degree of positioning accuracy in order to make precise observation of the sample. For example, in a SEM adapted to inspect a semiconductor wafer, the wafer is fixed on a stage which is arranged precisely at the position of observation in the electronic microscope so as to acquire an image of the wafer. At this stage, should the stage be positioned with errors, the position of observation by the electron microscope would be deviated. As a result, there have been desired a positioning device and a drive unit therefor, which are capable of positioning the stage with a high degree of accuracy.
Heretofore, there has been broadly used a positioning device in which a ball screw is rotated by a stepping motor so as to cause a nut mounted on the ball screw to displace a stage (which will be hereinafter referred to as “ball nut type”).
As to another type, instead of a conventional stepping motor, there has been a direct drive type positioning device in which an ultrasonic motor composed of a piezoelectric element (electrostrictive element) is attached directly to the side surface of the stage in order to directly displace the latter. Patent Document 1 discloses a drive system in which a piezoelectric element is pressed against a drive transmission surface attached to the side surface of the stage, and accordingly, the stage is rectilinearly moved in the direction of displacement of the stage with the use of the shearing deformation of the piezoelectric element applied thereto with a power voltage in order to displace the stage.